Update the spacewarp sample application from the 2020.3 branch to the latest Unity 2021 version (2021.3.6f1) and add it to the 2021.2 branch.

The project compiles and the resulting application demonstrates spacewarp and is usable in conjunction with a Quest 2 device.

Please read the Contributing guide before making a PR.

[ ] Have you added a backport label (if needed)? For example, the need-backport-* label. After you backport the PR, the label changes to backported-*.
[ ] Have you updated the changelog? Each package has a CHANGELOG.md file.
[ ] Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR. If you do add documentation, make sure to add the relevant Graphics Docs team member as a reviewer of the PR. If you are not sure which person to add, see the Docs team contacts sheet.
[ ] Have you added a graphic test for your PR (if needed)? When you add a new feature, or discover a bug that tests don't cover, please add a graphic test.

Why is this PR needed, what hard problem is it solving/fixing?

Creates sample spacewarp app for developers to experiment with the feature and use as an example for implementation in their own projects.

Describe what manual/automated tests were performed for this PR

Project was built and the functionality of the spacewarp feature it adds was verified.

Notes for the reviewers you have assigned.

Oculus-VR / Unity-Graphics